Fluorescent based optical sensors wherein a sensing membrane is layered onto a light transmissive substrate are known. The sensing membrane of the sensor is brought into contact with a sample while an excitation light reaches the sensing membrane through the substrate. The combination of the excitation light, the sensing membrane and a particular analyte will cause the sensing membrane to emit a fluorescing light. The emission signal from the sensing membrane is then detected through the light transmissive substrate from the back side of the sensor. Due to the fact that the sensing membranes of the sensor are quite thin there is a fairly large amount of the excitation light which passes through the sensing membrane and into the sample or into the sample chamber. The light which passes through the sensing membrane may be scattered, absorbed or reflected by the sample or the chamber walls back into and through the sensing membrane. Additionally, the fluorescing signal emitted from the sensing layer, which is indicative of the detection of the amount of the analyte of interest of the sample under test, may also be absorbed, scattered or reflectedly the sample back to the detector. The scattering, absorbing or reflecting of the excitation light and the fluorescing light emitted by the sensing membrane can combine to provide a four fold change in the signal between a perfectly reflecting and perfectly absorbing signal, thus severely skewing the detection results of the sensor.
Previous attempts to address this issue of unintended light affecting the results of the sensor include coating the sensing membrane with a support layer material which has been impregnated with a second material, or coating the sensing membrane with a plurality of layers such that the amount of light escaping the sensor into the sample and sample chamber is a very small fraction of the total light directed to the sensor. These attempts utilized a complex chemical process to produce an opaque, chemically permeable multilayered structure which is then laminated onto the sensing membrane. For example, U.S. Pat. No. 5,091,800 discloses the construction of an ion permeable cover membrane formed from a cross linked PVOH or cellophane substrate which is stretched onto a form and impregnated with silver, gold or platinum colloidal precipitants through a series of chemical treatments to form the opaque membrane. U.S. Pat. Nos. 5,081,041 and 5,081,042 disclose the use of an ion permeable cover membrane fabricated from a Dextran or cellulose substrate and impregnated with detergent solvated carbon black. U.S. Pat. Nos. 4,919,891 and 5,075,127 utilize cellulose acetate/acetone mixtures of either copper pthalocyanine or carbon black cast as separate coating membranes. U.S. Pat. No. 3,992,158 discloses the incorporation of a separate TiO.sub.2 -containing cellulose acetate for opacity or reflectance to be used in absorbance based chemistries on dry slides. Similarly, U.S. Pat. Nos. 4,042,335, 4,781,890, 4,895,704 and EP 0 142 849 B1 disclose the use of light blocking layers incorporating TiO.sub.2 particles for slide based chemistry tests. Such techniques have proven to be complex, labor intensive and expensive, requiring the utilization of multiple components or multiple layers of materials. It would be desirable to provide an inexpensive and simple to produce sensor including a single light attenuating layer of material deposited directly on the sensing membrane which reflects excitation and emission light back into the sensor without the light being affected by the sample while permitting the analyte of interest to freely diffuse through the light attenuating layer and into the sensing membrane.